"As scientists have described, (this is) the most significant variant they've encountered to date," British Transport Secretary Grant Shapps Shapps told Sky News.
The World Health Organization (WHO) is to hold a meeting in Geneva at 1100 GMT. Experts will discuss the risks it presents and if it should be designated as a variant of interest or variant of concern, WHO spokesperson Christian Lindmeier said.
Nearly 100 sequences of the variant have been reported, and early analysis shows it has "a large number of mutations" requiring further study, Lindmeier said.
One epidemiologuist said it may be too late to tighten travel curbs.
"I think we have to recognise that most likely this virus is already in other places. And so if we shut the door now, it’s going to be probably too late," said Ben Cowling of the University of Hong Kong.
Scientists are still learning about the variant, first identified at the start of this week, but the news pummelled financial markets, with stocks in Asia on Friday suffering their sharpest drop in three months and oil plunging more than 3%.
"Fear has gripped the financial markets with the travel industry flying into another violent storm," said Susannah Streeter, senior investment and markets analyst with Hargreaves Lansdown.
The variant, called B.1.1.529, has also been found in Botswana and Hong Kong, according to the UK Health Security Agency.
India issued an advisory to all states to test and screen international travellers from South Africa and other "at risk" countries after easing some of its travel restrictions earlier this month. Singapore's health ministry said it would also restrict arrivals from South Africa and countries nearby as a precaution.
Italy imposed an entry ban on people who have visited a group of southern African states, including South Africa, in the last 14 days.
Berlin will also declare South Africa a virus variant area after the detection of the new variant there, a health ministry source said.
European countries had already been expanding booster vaccinations and tightening curbs as the continent battles a fourth wave of the coronavirus, led by the delta variant, with many reporting record daily rises in cases.
The new wave comes as Europe and the United States enter winter, with more people gathering indoors in the run-up to Christmas, providing a perfect breeding ground for infection.
Germany reported a record of more than 76,000 COVID-19 infections in a day as its air force got ready for the first time in the pandemic to fly severely ill patients to other parts of the country to unburden struggling hospitals.
The coronavirus has swept the world in the two years since it was first identified in central China, infecting almost 260 million people and killing 5.4 million.
Japan also tightened border controls for visitors from South Africa and five other African countries, the Jiji news service reported. Its foreign ministry did not immediately respond to a Reuters request for comment.
Taiwan said travellers from "high-risk" southern African countries will have to go into quarantine.
New Zealand Prime Minister Jacinda Ardern said the country was well prepared for the new variant, days after saying it would reopen its borders to fully vaccinated international travellers from April 30.
(Reporting by Chen Lin in Singapore, Neha Arora in New Delhi, Rocky Swift in Tokyo, Ben Blanchard in Taipei, and Jane Wardell in Sydney, Aleksander Solum in Hong Kong and bureaux across the world; Writing by Miyoung Kim and Nick Macfie; Editing by Philippa Fletcher and by John Stonestreet) ((firstname.lastname@example.org;))